Method of forming a sheet metal door

ABSTRACT

A method of forming a sheet metal door with intersecting stiffener ribs so as to rigidify the door and prevent it from flexing along mutually perpendicular axes by cutting slit means at each corner of a sheet of metal, each slit means being defined by a pair of spaced diverging side edges originating at a common point spaced interiorly from the outer edges of the sheet, forming an elongated rib shaped depression in the sheet parallel to each outer edge of the sheet and along an axis that intersects one of the side edges of a pair of slit means intermediate the ends thereof so as to cause the side edges of each slit means to move out of the plane of the sheet towards each other while maintaining the outer edges in the plane of the sheet, and finally welding the side edges of each slit means together.

O United States Patent 1 1 [111 ,848,324

Perger Nov. 19, 1974 METHOD OF FORMING A SHEET METAL DOOR Primary Examiner-C. W. Lanham Assistant Examiner-Victor A. DiPalma [75] Inventor. Julius M. Perger, Cleveland, OhlO Attorney Agent, or Firm Edward J. Biskup [73] Assignee: General Motors Corporation,

Detl0lt, MlCh. 57 ABSTRACT Filed: p 4, 1973 A method of forming a sheet metal door with inter- [211 Appl No; 394 160 secting stiffener ribs so as to rigidify the door and prevent it from flexing along mutually perpendicular axes by cutting slit means at each corner of a sheet of [52] US. Cl 29/457, 29/476, meta], each slit means being defined by a pair Of 113/1 M, 1 113/116 Y spaced diverging side edges originating at a common [51] lilt- Cl point paced interim-[y from the outer edges of [he [58] Fle of Sear sheet, forming an elongated rib shaped depression in 29/457, l13/1 1 116 116 the sheet parallel to each outer edge of the sheet and 116 1 16 Y along an axis that intersects one of the side edges of a pair of slit means intermediate the ends thereof so as References Cited to cause the side edges of each slit means to move out UNITED STATES PATENTS of the plane of the sheet towards each other while 1,594,609 8/1926 Frease 113 116 F maintaining the Outer edges in the Plane of the Sheet, 2,485,648 10/1949 Norquist 29/476 x and finally Welding the Side edges of each Slit means 2,641,825 6/1953 Adams.... 29/476 together. 2,869,228 l/l959 Martin 29/475 X 3,388,446 6/1968 Philips 29/457 x 4 13 Draw Flgm'es zzz l w 2.21; '1 m K l 200 ,5,

' fizz/i L I!!! mp c l y I l ma 1 a? The cab doors used with off-highway earth moving vehicles such as dump trucks, front end loaders, scrapers and the like are made from sheet metal panels which are stiffened by the addition of elongated bars fastened to the inner side of the panel. The bars are rectangular in cross section and are welded to the panel at points along their lengths parallel to and spaced from each side edge while the parts are held in a fixture. As should be apparent, this method of stiffening the door panel has certain drawbacks not the least of which is that it requires the bar stock to be pre-cut to size, located, and tack welded to the panel. As a consequence, the cost of manufacturing the door is relative high due to the added material used and the labor required for fixturing the sheet metal and bar'stock and performing the welding operations. A further difficulty with forming doors in the above manner is that the tack welds used cause distortions in the outer surface of the panel resulting in a door which is unattractive from an ap pearance standpoint.

Accordingly, the objects of the present invention are to provide a method of forming a door having stiffener ribs that extend along intersecting axes; to provide an inexpensive method of forming a door with stiffening ribs of a desired depth in a relatively thick sheet metal panel without causing wrinkles or distortions in the panel; to provide a method of making a sheet metal door with separate corrugations that meet at a common juncture and are joined together by a welding operation; and to provide a method of forming a sheet metal door with a plurality of integral stiffening sections that are located along each side edge of a rectangular sheet metal panel with the stiffener sections intersecting each other and being formed in the panel by a cold-forming operation.

The above objects and others are realized in accordance with the invention by a method which forms intersecting stiffener sections in a flat rectangular sheet of deformable metal by first cutting or otherwise forming slit means at each corner of the sheet. Each slit means is defined by a pair of spaced diverging side edges of equal length originating at a common point spaced interiorly from the outer edges of the sheet. An elongated depression shaped as a rib or corrugation is then formed in the sheet parallel to each outer edge thereof and along an axis that intersects one of the side edges of a pair of slit means intermediate the ends thereof so as to cause the side edges of each slit means to move out of the plane of the sheet towards each other and meet at a juncture while maintaining the outer edges in the plane of the sheet. Afterwards, the side edges of each slit means are joined together by a welding operation or the like so as to cause the depressions to be rigidly interconnected and serve as stiffener ribs for the sheet.

A more complete understanding of the invention will be derived from the following detailed description when taken with the drawings in which:

FIG. 1 is an isometric view showing a sheet metal panel incorporating stiffener ribs of the type that are used in the present invention;

FIG. 2 is a plan view of a sheet metal blank having guide lines marked thereon and slit means formed therein preparatory to the formation of the stiffener ribs in the panel of FIG. 1;

FIG. 3 is an elevational view of a portion of a press brake incorporating a V-die for making one of the stiffener ribs in the panel of FIG. 2;

FIG. 4 is a view taken on line 44 of FIG. 3 showing the shape of the V-die used in making one of the stiffener ribs;

FIG. 5 is a cross-sectional view taken on line 55 of FIG. 3;

FIG. 6 is a view similar to that of FIG. 5 but shows the press brake closed for forming one of the ribs in the sheet metal blank;

FIG. 7 is a view similar to that of FIG. 2 but shows the stiffener rib formed in the blank by the dies of FIGS. 3 and 4;

FIGS. 8 and 9 areplan views of the V-dies utilized in making the other two stiffener ribs in the sheet metal blank of FIG. 2;

FIG. 10 is an enlarged view of the X-shaped slit means formed in the blank of FIG. 5;

FIG. 11 is a sheet metal blank of a vehicle door that is to be formed with ribs in accordance with the present invention;

FIG. 12 is an enlarged view of one of the corners of the sheet metal blank of FIG. 11; and

FIG. 13 is a perspective view showing one corner of the sheet metal blank of FIG. ll after the ribs are formed in the blank.

Referring to the drawings and more particularly FIG. 1 thereofa flat sheet metal panel 10 is shown having intersecting V-shaped stiffener ribs formed therein. As seen, the panel 10, in general, consists of a flat sheet metal portion 11 in which a stiffener rib 12 extends along the longitudinal axis thereof. In addition. a pair of transverse stiffener ribs 14 and 16 are provided in the portion 11 one of which is normal to and crosses the rib 12 while the other is located at one end thereof. The ribs 12, 14 and 16 serve to rigidify the sheet metal portion 11 by providing stiffener sections in two directions.

As seen in FIG. 2, the sheet metal portion 11 is shown as a blank prior to having the ribs 12, I4, and I6 formed therein and it will be noted that it is a flat rectangular sheet of steel that has a uniform thickness and is marked with guide lines for locating the positions of the stiffener ribs l2, l4, and 16. The guide lines are marked on one surface of the sheet metal blank and can be made using any form of scoring tool or marking instrument which will provide identifying lines that can be readily seen. As should be apparent, the guide lines serve to outline the area where the ribs are to be formed and in this regard it will be noted that parallel lines 18, 20, and 22 locate the position for rib 12. The spacing between the guide lines is determined by the type and size of die utilized for forming the rib into the panel. In this instance, as seen in FIG. I the rib 12 as well as ribs 14 and 16 are identical in the form and have a V-shape provided by the V-dies of FIGS. 4, 8 and 9 which are identical in cross-section. Accordingly, the distance between the outer guide lines 18 and 22 will be substantially equal to the sum of the transverse dimensions of both faces on.each V-die. Preferably, the ribbing will be of uniform cross-section and therefore, the distance between the outer guide lines for each rib to be formed will be the same. This is the case with the ribs 12, 14 and 16 in the panel 10 of FIG. 1. Accordingly, the location of the stiffener rib 14 is marked by parallel guide lines 24, 26, and 28 while the location of the stiffener rib 16 is identified by the parallel guide lines 30, 32 and 34.

After the blank has been marked with the guide lines described above, the blank is formed with slit means as seen in FIG. 2. Thus, at the intersecting outlined area for ribs 12 and 14, an X-shaped opening is cut into the sheet metal blank while at the connecting outlined area for ribs 12 and 16, a Y-shaped opening is cut into the sheet metal. Both the X- and Y-shaped openings are actually composed of two or more slit means and, in this regard, it will be noted that the X-shaped opening is made up of four slit means while the Y-shaped opening is formed with two slit means. In each case, the slit means is identical in configuration and is identified by reference numeral 36.

More specifically, the X-shaped opening is formed with two pairs of aligned slit means 36 with one pair lying along an axis which extends between the points of intersection of guide lines 18 and 24 and guide lines 22 and 28 and the other pair lying along an axis that extends between the points of intersection of guide lines 22 and 24 and guide lines 18 and 20. Each slit means 36 originates at the point of intersection of the outer guide lines and is defined by a pair of diverging side edges 38 and 40 of equal length having an included angle that is bisected by the axis along which the slit means is formed.

The slit means 36 can be cut into the sheet metal blank with a saw or any other well known tool for cutting openings in sheet metal along thin lines. The size of the included angle between the side edges 38 and 40 of each slit means 36 will depend upon the type of stiffener rib that is being formed in the sheet metal blank, the object being that upon forming the intersecting ribs, the side edges 38 and 40 of each slit means will meet at substantially the same elevation to permit the ribs to be joined together as will be described hereinafter.

an axis extending between the point of intersection between outer guide lines 18 and 30 and the point of intersection of lines and 32. The other slit means 36 lies along an axis extending between the point of intersection of lines 30 and 22 and the point of intersection of lines 20 and 32. In addition, a slot is formed in the blank starting at the point of intersection of lines 20 and 32 and along an axis in line with the guide line 20. The slot is defined by a pair of side edges 42 and 44 equally spaced from and parallel to the latter mentioned axis and constitutes the base leg of the Y-shaped opening.

The Y-shaped opening can be cut into the sheet metal blank in the same manner as the X-shaped opening. Thus, the slit means 36 will originate at the point of intersection of outer guide lines 18 and 30 in one case and at the point of intersection of guide lines 22 and 30 in the other case. As with the slit means used for making the X-shaped opening, each slit means will be however, will connect with the side edges 42 and 44 to complete the letter Y configuration.

With the sheet metal blank provided with the X- and Y-shaped openings as described above, the blank is then ready to have the stiffener ribs 12, 14, and 16 formed therein. In forming the stiffener ribs into the blank, a press brake can be used of the type shown in FIG. 3. As is well known, a press brake is a long power press that will accommodate various shapes of dies. In this case, the upper movable portion 46 of the press brake rigidly supports a male 90 V-die 48 which, as seen in FIGS. 4 and 5, has an outer configuration that will be used for shaping the stiffener rib 12. In crosssection the V-die 48 comprises inclined faces or surfaces 50 and 52 which respectively terminate at horizontally aligned flat surfaces 54 and 56 of the press brake portion 46. In plan view, as seen in FIG. 4, the V-die 48 is provided with V-shaped cut-out portions 58 and 60 which serve as clearance space for the side edge 40 of the rib l4 and also terminates with a V-shape to provide clearance for the side edges 40 of the Y-shaped opening. The lower stationary portion 62 of the press brake rigidly supports a female V-die 64 which as seen in cross section in FIG. 5, conforms to the shape of the male V-die member 48. The V-die has inclined surfaces or faces 66 and 68 which terminate at horizontally aligned fiat surfaces 70 and 72 so that upon closing the press, the inclined surfaces and the fiat surfaces of the male and female V-dies register with each other.

In forming the ribs I2, 14, and 16, the blank of FIG. 2 is first located in the press brake in a position where the guide line 20 will be vertically aligned with the center longitudinal axis or apex of the male V-die 48. In addition, the center of the X-shaped opening in the blank is positioned in vertical alignment with the point of intersection of the two cut'out portions 58 and 60 in the V-die 48. Once the blank is properly positioned, the upper portion 46 of the press is powered downwardly causing the sheet metal between the outer guide lines l8 and 22 to be bent out of the plane of the sheet to form the V-shaped stiffener rib 12 shown in- FIGS. 6 and 7. During the forming operation, and as seen in FIG. 10, the side edge 40 of each slit means 36 moves inwardly towards the other side edge 38. This occurs because during the forming operation, the V-die 48 contacts the upper surface of the sheet metal blank and depresses a portion thereof into the female V-die 64. Inasmuch as the rib forming operation is not a stretching or drawing operation but a bending operation, the sheet on opposite sides of the guide line 20 is moved towards the guide line 20. This causes the side edge 40 of each slit means 36 incorporated with the X- and Y- shaped openings to move in the direction of the adjacent side edge 38. In addition, the side edges 42 and 44 of the base leg of the Y-shaped opening move into substantial engagement as seen in FIG. 7.

The press is then opened and the sheet metal blank is removed therefrom and turned for positioning at another location of the press brake that is provided with a male V-die 74 that in plan view has the configuration shown in FIG. 6. The V-die 74 is identical in size to V-die 48 and includes V-shaped cut-out portions 76 and 78 which will serve to provide clearance space for the pointed ends defined by side edges 38 of rib l2 during the forming of rib 14. The procedure for forming rib 14 is the same as that used for forming rib l2, and it will be noted that during the forming of rib 14 by V-die 74, the sheet metal on the opposite side of the outer guide lines 24 and 28 moves towards the guide line 26 and the side edge 40 of each slit means 36 of the X-shaped opening moves upwardly in the direction of the other side edge 38 and meets the latter at substantially the same elevation relative to the plane of the sheet metal blank.

Afterwards, the sheet metal blank is positioned at another location of the press brake that has the upper movable portion thereof provided with a V-die 80 seen in plan view in FIG. 9. The V-die 80 is used for forming rib l6 and has a V-shaped cut-out portion 82 that provides clearance space for the pointed end of rib 12. The sheet metal blank is positioned with the guide line 32 located in vertical alignment with the longitudinal center of the V-die 74. in addition, the point of intersection of side edges 38 of the Y-shaped opening is located in vertical alignment with the apex of the V-shaped cutout portion 82 in the V-die 74. Once again the press is closed causing the sheet metal on opposite sides of guide line 32 to move towards each other and the side edges 40 to move upwardly towards the side edges 38 and meet at the substantially same elevation so as to form the stiffener rib 16 of FIG. 1.

The above described operations will result in the sheet metal panel having the stiffener ribs 12, 14, and 16 formed therein as shown in FIG. 1. In order to assure that the panel 10 will maintain a rigid form, the side edges 38 and 40 of each slit means are joined together. The preferred method of joining the side edges 38 and 40 is to weld them together resulting in a weld seam 84 being formed at each slit means 36 as well as at the base leg portion of the Y-shaped opening. After the welding operation has been completed, the panel is one integral member having the stiffener ribs l2, l4 and 16 formed therein so as to increase the flexural strength of the panel along axes which in this instance are mutually perpendicular to each other. As should be apparent, other methods of joining the side edges 38 and 40 together can be employed the object being to rigidly interconnect the sheet metal adjacent the side edge so that the ribs are integrally connected and serve to rigidify the panel along two directions. Also, although the stiffener ribs are formed in the panel along perpendicular axes the present method can be utilized for forming intersecting stiffener ribs at various angles.

FIG. 11 shows a sheet metal blank 86 that can be formed with stiffener ribs of the type described above and afterwards be used. as a closure member or door. The sheet metal blank 86 has the outer form of a rectangle and also has a rectangular opening 88 formed therein which can be used for supporting a window. The blank is formed of a plain flat sheet of steel which, because of the size of the blank, is relatively flexible. In order to rigidify the blank, the latter can be provided with ribs in accordance with the invention. In this regard, the desired location for the ribs would be to place them parallel to and equally spaced from the straight outer edges 90, 92, 94 and 96 of the blank. Accordingly, the surface of the blank is scored with guide lines 98, 100, 102 for a rib parallel to edge 90, guide lines 104, 106, 108 for a rib parallel to edge 92, guide lines 110, 112, 114 for a rib parallel to edge 94, guidelines 116, 118, 120 for a rib parallel to edge 96. As explained hereinbefore, the spacing between the guide lines for each rib will be determined by the size and type of forming die that will be used for bending the metal and forming the depression therein.

Slit means 122 are then formed at each corner of the blank. In this case. the type of ribs to be formed in the blank are the same and the spacing ofeach rib from the adjacent edge is equal. and therefore the slit means 122 in each corner will be identical. More specifically. each slit means 122 at each corner will lie along a central axis which will pass through the points of intersection of the inner, center and outer guide lines and the adjacent corner. Thus, the slit means 122 in the upper right corner of the blank will have a central axis 123 which passes through the points of intersection of guide lines 98 and 104, 100 and 106. 102 and 108 and the adjacent corner. The slit means 122 is defined by a pair of diverging side edges 124 and 126 of equal length which begin at the point of intersection of guide lines 98 and 104. The included angle of the slit means 122 is bisected by the axis 123 and as mentioned before, the size of the included angle will be determined by the type of rib that is to be formed in the blank and the thickness of the sheet metal that is being used.

The slit means 122 is formed in each corner of the blank as explained above and is cut therein by using a saw ,or any other method used for making cuts along thin lines in sheet metal. After cutting the slit means 122 in each corner of the blank. the latter is ready to be placed in a press brake for forming the ribs. in this instance, the ribs to be formed in the blank are V- shaped and accordingly V-dies such as those used for fonning the panel 10 can be used. The same V-die can be used for forming the rib along the edges 92 and 96 and, as should be apparent, will have a length that will conform to the distance bounded by the guide lines 104 and 108 and between the side edges 126 of the upper and lower slit means along edge 92. The ends of the V-die will be angled so as to provide clearance for the side edges 124 and thereby assure that only the metal between the side edges 126 of the upper and lower slit means 122 is subjected to the bending operation. Similarly, the same V-die will be used for forming the ribs along edges and 94 and the length of the V-die will correspond to the distance between the side edges 124 along edge 90. The latter V-die will also have the ends thereof angled so as to provide clearance for the side edges 126 and assure that only the metal between the side edges 124 along edge 90 and also edge 94 is'subjected to the bending operation.

Assuming the ribs along edges 90 and 94 are to be formed first, the blank will be positioned in the press brake so that the guideline is vertically aligned with the longitudinal center of the V-die and the side edges 124 of the slit meansare vertically aligned with the angled ends of the V-die. The press is then closed causing the sheet metal on opposite sides of the guide line 100 to move towards each other with resultant movement of the side edge 126 of each upper slit with the proper V-die therein for forming the ribs along the edges 92 and 96. Again the center-guide lines and the connecting side edges are used for obtaining correct alignment of the blank relative to the V-die. Upon closing the press, the sheet metal on opposite sides of the center guide line will move towards each other and the side edges 126 between the inner and outer guide lines will move upwardly to meet the side edge 124 at substantially the same elevation. As a result each corner of the blank will have intersecting ribs such as the ribs indicated by reference numerals 128 and 130 in FIG. 13. Afterwards, the side edges 124 and 126 of each slit means 122 can be joined together by a welding operation which will provide a weld seam 132 with the outer edges 90, 92, 94 and 96 lying in the plane of the blank.

A blank successfully formed into a door as described above was made from a hot rolled commercial quality pickled in oil sheet steel, 0.20 percent maximum carbon and measured 1 1 gauge or 0.1 196 inches in thickness. The blank was inches in width and was 50.88 inches long. The V-die used for forming the depressions or ribs in the blank had an included angle of 90 with each surface of the V having a transverse dimension of 1.25 inches. Accordingly, the spacing between the inner and outer guide lines along the edges 90, 92, 94 and 96 measured 2.50 inches and the outer guide line in each case was placed approximately 1.25 inches from the adjacent edge of the blank.

If desired a rib could be placed in the aforementioned door blank slightly below the window opening 88, parallel to the edges 90 and 94, and interconnecting the ribs 130 formed along edges 92 and 96. In such case, the blank would be marked with guide lines as explained above and the area of intersection of the guide lines along side edges 96 and 92 with the cross rib would each be provided with a Y-shaped opening as explained in connection with the panel 10.

Finally, it will be noted that prior to forming the door as explained above, a relatively small drilled hole can be placed at the originating point of the slit means 122. The hole will serve to relieve any stresses that may occur in the metal at the aforementioned point during the forming of the ribs.

Various changes and modifications can be made in this construction without departing from the spirit of the invention. Such changes and modifications are contemplated by the inventor and he does not wish to be limited except by the scope of the appended claims.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

.1. A method of fabricating a closure member comprising the steps of, providing a flat sheet metal panel having a plurality of interconnected outer edges form ing a plurality of corners, cutting slit means in each corner of said sheet metal panel so as to form a pair of spaced side edges lying on opposite sides of a line that intersects the corner, forming an elongated depression in said panel adjacent to and spaced from each outer edge and along an axis that intersects the side edges of a pair of slit means intermediate the ends thereof so as to cause the side edges of each slit means to move out of the plane of the panel towards each other and meet plane of the panel, and permanently connecting the side edges of each slit means together so that each depression serves as a stiffener means for rigidifying said panel.

2. A method of fabricating a closure member having stiffener ribs comprising the steps of, providing a flat sheet metal panel having a plurality of interconnected outer edges forming a plurality of corners, placing openings in each corner of said sheet metal panel defined by a pair of diverging side edges originating at a point spaced interiorly from said outer edges and lying on opposite sides of a line that intersects the comer. corrugating the sheet metal panel in localized areas thereof so as to form an elongated depresssion in said panel adjacent to and spaced from each outer edge thereof and along an axis that intersects the side edges of a pair of slit means intermediate the ends thereof so as to cause the side edges of each slit means to move out of the plane of the panel towards each other and meet at ajuncture while maintaining the outer edges in the plane of the panel, and permanently joining the side edges of each slit means together so that each depression serves as a stiffener rib for rigidifying said panel.

3. A method of fabricating a door with stiffener ribs comprising the steps of, providing a flat sheet metal panel having straight outer edges forming a plurality of corners, forming slit means in each corner of said sheet metal panel so as to provide a pair of spaced side edges originating at a point spaced interiorly from said outer edges and lying on opposite sides of aline that intersects the corner, bending the sheet metal panel in localized areas so as to form an elongated corrugation in said panel adjacent to and spaced from each outer edge thereof and along an axis that intersects the side edges of a pair of slit means intermediate the ends thereof so as to cause the side edges of each slit means to move out of the plane of the panel towards each other and meet at a juncture while maintaining the outer edges in the plane of the panel, and welding the side edges of each slit means together so that each corrugation serves as a stiffener rib for rigidifying said panel.

4. A method of fabricating a door with stiffener ribs comprising the steps of, providing a rectangularly shaped flat sheet metal panel having an opening for a window and four straight outer edges forming four corners, cutting slit means in each corner of said sheet metal panel so as to form a pair of diverging side edges of equal length originating at a point spaced interiorly from said outer edges and lying on opposite sides of a line that bisects the included angle of the corner, forming an elongated V-shaped depression in said panel parallel to and spaced from each outer edge thereof and along an axis that intersects the side edges of a pair of slit means intermediate the ends thereof so as to cause the side edges of each slit means to move out of the plane of the panel towards each other and meet at a juncture while maintaining the ,outer edges in the plane of the panel, and welding the side edges of each slit means together so that each V-shaped depression serves as a stiffener means for rigidifying said panel. 

1. A method of fabricating a closure member comprising the steps of, providing a flat sheet metal panel having a plurality of interconnected outer edges forming a plurality of corners, cutting slit means in each corner of said sheet metal panel so as to form a pair of spaced side edges lying on opposite sides of a line that intersects the corner, forming an elongated depression in said panel adjacent to and spaced from each outer edge and along an axis that intersects the side edges of a pair of slit means intermediate the ends thereof so as to cause the side edges of each slit means to move out of the plane of the panel towards each other and meet at a juncture while maintaining the outer edges in the plane of the panel, and permanently connecting the side edges of each slit means together so that each depression serves as a stiffener means for rigidifying said panel.
 2. A method of fabricating a closure member having stiffener ribs comprising the steps of, providing a flat sheet metal panel having a plurality of interconnected outer edges forming a plurality of corners, placing openings in each corner of said sheet metal panel defined by a pair of diverging side edges originating at a point spaced interiorly from said outer edges and lying on opposite sides of a line that intersects the corner, corrugating the sheet metal panel in localized areas thereof so as to form an elongated depresssion in said panel adjacent to and spaced from each outer edge thereof and along an axis that intersects the side edges of a pair of slit means intermediate the ends thereof so as to cause the side edges of each slit means to move out of the plane of the panel towards each other and meet at a juncture while maintaining the outer edges in the plane of the panel, and permanently joining the side edges of each slit means together so that each depression serves as a stiffener rib for rigidifying said panel.
 3. A method of fabricating a door with stiffener ribs comprising the steps of, providing a flat sheet metal panel having straight outer edges forming a plurality of corners, forming slit means in each corner of said sheet metal panel so as to provide a pair of spaced side edges originating at a point spaced interiorly from said outer edges and lying on opposite sides of a line that intersects the corner, bending the sheet metal panel in localized areas so as to form an elongated corrugation in said panel adjacent to and spaced from each outer edge thereof and along an axis that intersects the side edges of a pair of slit means intermediate the ends thereof so as to cause the side edges of each slit means to move out of the plane of the panel towards each other and meet at a juncture while maintaining the outer edges in the plane of the panel, and welding the side edges of each slit means together so that each corrugation serves as a stiffener rib for rigidifying said panel.
 4. A method of fabricating a door with stiffener ribs comprising the steps of, providing a rectangularly shaped flat sheet metal panel having an opening for a window and four straight outer edges forming four corners, cutting slit means in each corner of said sheet metal panel so as to form a pair of diverging side edges of equal length originating at a point spaced interiorly from said outer edges and lying on opposite sides of a line that bisects the included angle of the corner, forming an elongated V-shaped depression in said panel parallel to and spaced from each outer edge thereof and along an axis that intersects the side edges of a pair of slit means intermediate the ends thereof so as to cause the side edges of each slit means to move out of the plane of the panel towards each other and meet at a juncture while maintaining the outer edges in the plane of the panel, and welding the side edges of each slit means together so that each V-shaped depression serves as a stiffener means for rigidifying said panel. 